DDT (2,2 bis-(p-chlorophenyl)-1,1,1-trichloroethane) and its degradation products are today found almost everywhere in the global environment. Numerous studies also report on the accumulation of often relatively high concentrations of environmental pollutants like PCB, dioxins and brominated flame retardants, and pesticides like toxaphenes and DDT and its metabolites in the deposit of e.g. marine organisms. The hazard of these compounds for both humans and animals have caused a growing concern about the content of toxic substances in food and food stuff. Consumption of dioxins above safe levels over a lifetime may result in an increased risk of cancer.
Food products that have no or reduced amounts of pollutants are gaining popularity as well as an increasing share of the market. Consequently, removal or reduction of pollutants in food products have the potential to substantially increase marketability and value.
The commercially important polyunsaturated fatty acids in marine oils, such as fish oil, are preferably EPA (eicosapentaenoic acid, C20:5n-3) and DHA (docosahexaenoic acid, C22:6n-3). The full nomenclature of these acids according to the IUPAC system is: EPA all-cis-5,8,11,14,17-eicosapentaenoic acid, DHA all-cis-4,7,10,13,16,19-docosahexaenoic acid. For many purposes it is necessary that the marine oils should be refined in order to increase the content of EPA and/or DHA to suitable levels, or to reduce the concentrations of, or even eliminate, certain other substances which occur naturally in the raw oil.
The fatty acids EPA and DHA are also proving increasingly valuable in the pharmaceutical and food supplement industries in particular. It is also very important for fish oils and other temperature sensitive oils (e.g. oils that contain long chain polyunsaturated fatty acids) to keep the temperature in some of the processes as low as possible.
The demand for marine oils of high quality is increasing. This issue forces the fish oil industry to consider use of alternative refining techniques for fish oils with inferior quality, i.e. oils with high amounts of free fatty acids that make the oils less useful for nutritional purposes and make traditional alkaline refining more complicated and costly. If environmental pollutants can be successfully removed from such fish oils they are appropriate for use in the animal feed industry, e.g. in animal feed products.
From the literature it is known that molecular distillation, or short path distillation as the technique alternatively may be named, can be used to remove the pesticides DDT and its metabolites from fish oil (K. Julshamn, L. Karlsen and O. R. Braekkan, Removal of DDT and its metabolites from fish oils by molecular distillation, Fiskeridirektoratets skrifter; Serie teknologiske undersøkelser, Vol. 5 No. 15 (1973)). A practical upper limit was 65% removal together with a loss of about 25% of vitamin A. In many industrial fish oil refining processes a removal of DDT up to 65% is not satisfactory.
Anthony P. Bimbo: Guidelines for characterization of food-grade fish oil. INFORM 9(5), 473-483 (1998), reported that vacuum stripping or thin-film distillation can be used to remove chlorinated hydrocarbons and free fatty acids from fats or oils. A disadvantage by using vacuum stripping to refine oils is that sufficient results only can be achieved then the vacuum stripping process is carried out at a high temperature. Further, the high temperature gives rise to undesirable side reactions.
Jiri Cmolik og Jan Pokorny: Physical refining of edible oils, Eur. J. Lipid Sci. Technol. 102(7), 472-486 (2000) describes physical refining of edible oils and the use of molecular distillation for removal of undesirable substances in crude oils, preferably crude vegetable oils, respectively the use of steam stripping in order to remove free fatty acids from an oil composition. Physical refining is used to refine oils of good quality, i.e. oils with small amounts of free fatty acids. However, physical refining is more complicated and costly for oils with inferior quality.
In WO 9524459 a process for treating an oil composition containing saturated and unsaturated fatty acids in the form of triglycerides, in order to obtain a refined product with higher concentrations of the polyunsaturated fatty acids, is presented. This process also is intended to be used for removal of some environmental pollutants from an oil composition, wherein the process comprises the steps of; subjecting the oil composition to a transesterification reaction and thereafter subjecting the product obtained in the first step to one or more molecular distillations. This technique has the severe limitation that it can only be used for fish oils that have been partially transesterified using a lipase catalyst that discriminates against omega-3 fatty acids. Obviously, this technique can not be used for commercial fish oils.
In EP0632267 A1 a method of measuring the content of polycyclic aromatic hydrocarbons (PAH) remaining in lanolin is presented. The European patent document also describes a method of removing PAH remaining in wool grease or lanolin by a vacuum distillation of the grease or lanolin under specified conditions either directly or after having been treated with a borate and, if necessary, obtaining various lanolin derivatives from the treated wool grease or lanolin. However, the technique described in said patent document requires very high temperatures (230° C.) in order to achieve 90% reduction in PAH content.
Another interesting observation is that the removal of environmental pollutants from fats or oils is not a trivial matter. Several different techniques, some of which are mentioned above, to accomplish this task have been developed, but none of them is sufficiently effective and gentle to the fat or oil. In addition, it is nowadays a problem for e.g. the marine oil industry that the amounts of pollutants in e.g. fish oil become increased.